1. Field of the Invention
The invention relates to ammonia synthesis gas. More particularly, it relates to the purification of an ammonia synthesis gas stream.
2. Description of the Prior Art
Ammonia synthesis gas production is conventionally based on the steam reforming of natural gas or naphtha, followed by secondary reforming with air. It is also possible, however, to subject feedstocks to partial oxidation conversion, and there has been some trend toward basing commercial plants on the use of oxygen for the partial oxidation of heavier petroleum fractions and coal.
In the production of synthetic ammonia from hydrogen-nitrogen mixtures at elevated pressures, generally using iron or iron-based catalyst, the activity of the catalyst is reduced by the presence of oxygen-containing compounds. Almost invariably, the raw gas produced in the feedstock conversion operations referred to above contain such oxygen-containing compounds, namely carbon monoxide, carbon dioxide and water. Generally, raw synthesis gas purification processes thus are begun with the following two steps: (1) reaction of most of the carbon monoxide with steam for the conversion thereof to carbon dioxide and hydrogen in the so-called water gas shift conversion operation, and (2) removal of the bulk of the carbon dioxide by absorption, using suitable physical solvents, such as methanol and esters of oligoethylene glycols, or chemical-type solvents, such as hot potassium carbonate and solutions of amines. Following such treatment steps, the synthesis gas generally contains the following levels of impurities, expressed in mol percent: CO--between 0.2 and 1.0%; H.sub.2 O--between 0.1 and 0.2%; and CO.sub.2 --between 0 and 0.4%.
Further purification of said synthesis gas can be carried out employing caustic scrubbing and cryogenic techniques, generally preceded by adsorptive drying or purification techniques. A much more frequently employed technique, however, is a methanation operation, wherein the carbon oxides present in said synthesis gas react with hydrogen in accordance with the following reactions: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O, and (1) EQU CO.sub.2 +4H.sub.2 .fwdarw.CH.sub.4 +2H.sub.2 O. (2)
Most of the water vapor formed is removed in the after-methanation cooler, and residual amounts of water can be separated by adsorptive drying or, as is more widely employed, in the refrigeration part of the ammonia synthesis loop where it is dissolved in liquid ammonia. The methanation operation is carried out employing nickel-based catalyst at temperatures of between 200.degree. C. and 300.degree. C. It has the processing advantage that both CO and CO.sub.2 can be almost quantitatively reacted to form methane so that their residual concentrations are typically lower than 5 ppm.
The methanation reaction, however, also has disadvantageous features, such as the consumption of valuable hydrogen as indicated by reactions (1) and (2) above, and the formation of additional methane that accumulates in the ammonia synthesis loop as an inert gas.
Most frequently, the carbon monoxide concentration in the partly purified synthesis gas is within the range of from 0.3 to 0.6%, although it may be outside such range and typically within the broader range indicated above. It is recognized in that art that, upon completion of said water gas shift conversion, additional carbon monoxide can be removed from said synthesis by selective catalytic oxidation down to 0.01% (100 parts per million, ppm) or even lower. For this purpose, the raw gas after the shift reactions is cooled to below 80.degree. C., preferably below 50.degree. C., and at least as much air or oxygen is added thereto as is sufficient to react and oxidize 90-99% of the CO to CO.sub.2. This reaction is carried out using catalysts that selectively and preferentially enable oxygen to react with carbon monoxide, while the rate of reaction between hydrogen and oxygen is much slower, so that very little hydrogen is oxidized. This selective oxidation of carbon monoxide enables less hydrogen to be consumed in the methanation operation, so that less raw gas is needed for the overall operation and results in less methane formation and consequently less methane accumulation in the ammonia synthesis loop. The processing sequence in said approach thus comprises: water gas shift conversion of CO with steam to form CO.sub.2 ; conventional selective oxidation of additional CO with oxygen or air to form CO.sub.2, typically reducing the CO concentration of the partially purified gas stream to below 100 ppm CO; carbon dioxide removal by liquid absorption; conventional methanation; and drying.
While such processing has desirable benefits as indicated above, there is a continuing need in the art for the development of improvements to further reduce the energy requirements and the investment costs associated with said synthesis gas production. In particular, it would be highly advantageous in the art if the hydrogen consumption in a methanator would not only be further reduced, but if the methanator could be eliminated entirely. Such elimination of the methanator would reduce the investment cost of the overall operation significantly and would also result in a desirable reduction in the process pressure drops.
It is an object of the invention, therefore, to provide an improved process for the purification of ammonia synthesis gas.
It is another object of the invention to provide an ammonia synthesis gas purification process and system in which methanation of carbon oxides becomes unnecessary.
It is a further object of the invention to provide a lower cost process and system for the purification of raw ammonia synthesis gas.
It is a further object of the invention to provide a process and system for the production of dry synthesis gas having a low methane content, whereby the energy requirements of the ammonia synthesis operation are significantly reduced.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.